1. Field of the Invention
The invention relates to a method for calibrating a microwave measurement network having N terminals, N being an integer larger than one, the method comprising the steps of measuring open network parameter values of an open device having N terminals, which are mutually electrically insulated, measuring short network parameter values of a short device having N terminals, which are mutually electrically connected by conductors, each conductor having a short device DC resistance, measuring load network parameter values of a load device having N terminals, which are mutually electrically connected by resistors, each resistor having a load device DC resistance, which is larger than the short device DC resistance, the load device having a parasitic load impedance.
The invention also relates to a method for de-embedding a device under test having N terminals.
The invention also relates to a set of devices for de-embedding a device under test having N terminals.
The invention also relates to a vector network analyzer arranged for executing the method for de-embedding according to the invention.
2. Description of the Prior Art
U.S. Pat. No. 4,858,160 discloses an embodiment of a method for calibrating an N terminal microwave measurement network. The aim of this method is to calibrate an N terminal microwave measurement network such that the microwave characteristics of a device under test (DUT), which may be embedded in a semiconductor article of manufacture, can be determined by means of a vector network analyzer (VNA). The accurate knowledge of microwave characteristics of DUTs such as transistors and inductors is necessary when designing and manufacturing an apparatus applying microwave radiation such as, e.g., a mobile telephone.
A VNA comprises at least an input probe and an output probe, each probe having at least one signal lead. In many cases the probes also have a ground lead. The probes are brought into contact with bond pads, which are at the surface of the body comprising the DUT and which are electrically connected to the terminals of the DUT. The body comprising the DUT may be a semiconductor article of manufacture.
The VNA generates a microwave test signal, which is directed to the input probe. It measures the amplitude and phase of the signal reflected back into the input probe and the amplitude and phase of the signal transmitted to the output probe as function of, e.g., the frequency of the microwave test signal. The transmitted and the reflected signal are used to first determine the DUT network parameter values of the DUT, which are then used to determine the microwave characteristics of the DUT.
In general, the measurement of the microwave characteristics of the DUT is prone to artifacts due to parasitic impedances of various sources, including:
the VNA;
cables connecting the VNA and the probes;
the probes themselves;
the probes contacting the bond pads;
the bond pads themselves;
interconnects connecting the bond pads and the DUT.
In order to obtain the microwave characteristics of the DUT with a high precision, it is essential to remove the contributions of the parasitic impedances from the measurements. This is often done in two steps. In a first step the VNA is calibrated using a set of devices, which often comprises:
an open device, in which a bond pad to be contacted with an input lead is electrically insulated from a bond pad to be contacted with an output lead;
a short device, in which a bond pad to be contacted with an input lead is electrically connected to a bond pad to be contacted with an output lead in such a way that the DC resistance between the two bond pads is low, e.g. below 5 Ohm; and
a load device, in which a bond pad to be contacted with an input lead is electrically connected to a bond pad to be contacted with an output lead in such a way that the DC resistance between the two bond pads has a predetermined value of, e.g., 50 Ohm.
As a result of this calibration, ideally all parasitic impedances between the VNA and the probe tips, not including the bond pads and the interconnects, are determined and subsequently removed.
In a second step, which is often called de-embedding, it is aimed to remove the parasitic impedances of the bond pads and the interconnects from the measured network parameter values of the DUT. These parasitic impedances arm determined using another set of devices. All of the devices of this set are integrated in a semiconductor article of manufacture in a way similar to the DUT. Preferably, they all have bond pads and interconnects substantially identical to the corresponding parts of the DUT. This set of devices comprises:
the DUT;
an open device, in which a bond pad to be contacted with an input lead is electrically insulated from a bond pad to be contacted with an output lead;
a short device, in which a bond pad to be contacted with an input lead is electrically connected to a bond pad to be contacted with an output lead in such a way that the DC resistance between the two bond pads is low, e.g., below 5 Ohm; and
a load device, in which a bond pad to be contacted with an input lead is electrically connected to a bond pad to be contacted with an output lead in such a way that the DC resistance between the two bond pads has a designed value of, e.g., 50 Ohm.
After performing the calibration step, the reference plane of the microwave characterization is located at the bond pads. After performing the de-embedding step, the reference plane is located at the terminals of the DUT and the microwave characteristics of the DUT are obtained.
The accuracy of both the calibration step and the de-embedding step and, as a consequence, the accuracy of the determined microwave characteristics of the DUT depend critically on the exact knowledge of the impedances of the open device, the short device and the load device used for calibration and de-embedding.
It is a drawback of the known method for calibrating and de-embedding that the microwave characteristics of the DUT are determined not precisely enough, in particular at relatively high frequencies of, e.g., 10 GHz or higher.